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Abstract

We present a double-clad fiber coupler (DCFC) for use in confocal endomicroscopy to reduce speckle contrast, increase signal collection while preserving optical sectioning. The DCFC is made by incorporating a double-clad tapered fiber (DCTF) to a fused-tapered DCFC for achromatic transmission (from 1265 nm to 1325 nm) of > 95% illumination light trough the single mode (SM) core and collection of > 40% diffuse light through inner cladding modes. Its potential for confocal endomicroscopy is demonstrated in a spectrally-encoded imaging setup which shows a 3 times reduction in speckle contrast as well as 5.5 × increase in signal collection compared to imaging with a SM fiber.

Figures (5)

Confocal imaging with a DCF. (a) Light propagation schematic. Illumination light originates from the core of the DCF defined by its MFD (taken at the 1/e2 intensity point) and numerical aperture (illumination half-angle β). It is collimated with a lens of focal length fcoll and fills an objective lens (of focal length fobj, aperture 2aobj, illumination cone half-angle α). Light collection is performed through the inner cladding of the DCF (of diameter d). Light propagates along the z-axis and rf, rp and rs define the radial coordinates at the fiber, objective lens pupil and sample planes, respectively. (b) Optical sectioning (u1/2) for a perfect plane reflector as a function of the ratio d/MFD for different values of the pupil filling factor A. c) Excitation efficiency (η) for a perfect plane reflector at focus (u = 0) as a function of the ratio d/MFD for different values of A.

Double-clad tapered fiber (DCTF). (a) Schematic of the DCFC with a tapered end, the gray area representing a high index gel drop to diminish back reflections. (b) Schematic of the tapered end. (c) MFD (solid red line) and inner cladding diameter d (dotted black line) as a function of the taper ratio.

Simultaneously acquired images of a 7 day old mouse embryo fixed in a solution of 4% of paraformaldehyde. (a) Average of 20 images of the SM signal (scale bar of 50 μm). (b) Average of 20 images of the MM signal (scale bar of 50 μm). (c) Zoom of the SM image. (d) Zoom of the MM image. Intensity scale of the SM and MM images were normalized independently.